1. Field of the Invention
The present invention relates generally to fabrication of nuclear fuel rods and, more particularly, is concerned with a method of forming a gripper cavity in a fuel rod end plug.
2. Description of the Prior Art
In a typical nuclear reactor, such as a pressurized water type, the reactor core includes a large number of fuel assemblies each of which is composed of top and bottom nozzles with a plurality of elongated transversely spaced guide thimbles extending longitudinally between the nozzles and a plurality of transverse support grids axially spaced along and attached to the guide thimbles. Also, each fuel assembly is composed of a plurality of elongated fuel elements or rods transversely spaced apart from one another and from the guide thimbles and supported by the transverse grids between the top and bottom nozzles. The fuel rods each contain fissile material and are grouped together in an array which is organized so as to provide a neutron flux in the core sufficient to support a high rate of nuclear fission and thus the release of a large amount of energy in the form of heat. A liquid coolant is pumped upwardly through the core in order to extract some of the heat generated in the core for the production of useful work.
Each fuel rod includes nuclear fuel pellets and the opposite ends of the rod are closed by upper and lower end plugs to hermetically seal the rod. A cavity is formed in the lower end plug as part of the forming of the end plug. Subsequently, a groove is formed in the cavity by a secondary machining operation. The groove is provided in order to assist in assembling of the fuel assembly by insertion of the fuel rods into the grids of the fuel assembly. The groove in the cavity enables a gripping mechanism, such as disclosed in U.S. Pat. No. 4,966,745, assigned to the same assignee as the subject invention, to enter the lower end plug cavity, expand into the groove, and then pull the fuel rod at lower end plug through the grids of the fuel assembly already attached to the guide thimbles.
Several problems are associated with the current approach of machining the groove in the cavity. First, attaining the desired cavity and groove configuration requires an excessive amount of machining at a cost higher than the material cost of the bottom end plug itself. About one-half of the machining cost goes toward forming the groove in the cavity. Second, the fabrication of the groove is currently performed by an operator and thus depends on the operator doing it properly. However, occasionally the groove is left out completely due to operator oversight which creates fuel rod loading problems at final assembly. Third, sometimes the gripper mechanism shears out the material in the end plug due to high loading force and stress on the end plug.
Consequently, a need exists for a different approach to fabrication of the cavity and groove in the bottom end plug for the nuclear fuel rod so as to avoid the problems associated with the current techniques.